Unusual Brown/ blackish bleeding in with my regular cycle. Im sixteen and concerned? Whats wrong?

I turn sixteen in 2 weeks, My periods have always been generally normal since i started my first one. Its always been 6-7 days and really heavy arriving at the beginning or the end of the month. This month it came late. I got a really gross brown discharge when i should have started and its been over a week and i still have it except its darkened and is practically black. I have some pink and red mixed in which im assuming is blood but its a really light flow. I barely have to change my panti-liner and its not heavy enough for tampons. I usually have to double up with a super tampon and overnight pad combo worn throughout the day. I took a pregnancy test just in case.. well make that three. They all came out negative and they were first response so im sure its not a false negative. Ive had really terrible back aches, loss of sleep, off and on headaches, and cramps. its improved but im having mood swings and Some days the symptoms return really bad again. does anyone have any idea of what could be wrong? or if this could be normal? My mom is supposed to make an appointment but until then i just need an outsiders perspective.